Method of stripping aluminide coatings

ABSTRACT

A process for stripping aluminide coatings from metals and alloys of the iron group, by making the article to be stripped the anode in an electrolyte comprising an oxidizing acid, at a potential not higher than 2.4 volts.

nited States Patent [1 1 Scott METHOD OF STRIPPING ALUMINIDE COATINGS[75] Inventor: Bruce E. Scott, Wyckoff, NJ. [73] Assignee:Curtiss-Wright Corporation,

Wood-Ridge, NJ.

[22] Filed: Dec. 11,1972

[21] App]. No.: 313,873

[52] US. Cl. 204/146 [51] Int. Cl ..;L B01k 1/00 [58] Field of Search;204/146, 140

[56] References Cited UNITED STATES PATENTS 2,706,17l 4/1955 Goral204/146 2,840,521 6/1958 Wasserman' 204/146 3,260,660 7/1966 Brooks204/146 Primary Examiner-T. Tufariello Attorney-Raymond P. Wallace etal.

[57] ABSTRACT A process for stripping aluminide coatings from metals andalloys of the iron group, by making the article to be stripped the anodein an electrolyte comprising an oxidizing acid, at a potential nothigher than 2.4 volts.

9 Claims, 6 Drawing Figures 1 METHOD OF STRIPPING ALUMINIDE COATINGSBACKGROUND OF THE, INVENTION This invention relates to a method ofremoving a metallic coating from a basis metal, and more particularly toremoving aluminide coatings from basis metals of the iron group. Thealuminide coating may comprise an aluminide phase of the basis metal,and a surface phase of more or less pure aluminum.

The development of modern aircraft power plants has resulted in a demandfor metals capable of withstanding high operating temperatures, suchmetals being designated superalloys. In the main, they are alloys havinga nickel base, cobalt base, or nickel-cobalt base, although some alloyshaving an iron base are also used. Turbine blades fabricated of suchsuperalloys are exposed not only to high temperatures, but to the verycorrosive combustion gases of the turbine, and hence must have highoxidation and corrosion resistance. Although the superalloys inherentlyhave a high degree of such resistance, it is common practice to augmentit by aluminiding the surface of turbine blades, that is, diffusing analuminum coating into the surface of the blades. I

Aluminiding is performed by packing the blades in powdered aluminum witha suitable activator, and bringing the pack up to the fusing temperatureof the aluminum for an appropriate period of time. Aluminum diffusesinto the surfaceof the blade where it forms an aluminide bond with thebasis metal, both in an intergranular manner and into the matrix of thebasis metal. The aluminide phase is further overlaid with a phase ofpure or nearly pure aluminum.

In overhauling such engines it is necessary to strip the blades of bothphases of the aluminum coating, in order to examine the blades fordefects such as cracks, incipient cracks, or evidences of corrosionwhich would indicate a potential weakness of the basis metal. In theprior art such stripping has been done in an electrolytic cell, with theblade as anode in a hydrochloric acid solution and with a drivingpotential of 6-20 volts. Hydrochloric acid was used because of itsvigorous attack on aluminum, shortening the time of the process.However, since it is a reducing acidit also attacks the basis metalunless careful and continuous attention is given throughout the process.

If such careful control is not exercised there may be a serious effecton the dimensional integrity of the blade, such as surface attackproducing'roughness, intergranular attack with weakening of thestructure, rounding off of small radii, change in critical curvatures,and in the case of rotor blades, alterations of the dimensions of theblade root which controls the fit to the turbine rotor. Even thoughclose watch is kept on the process, intergranular corrosion of the basismetal may still occur, principally due to the strong reducing action ofthe acid, which will result in eventual failure of the blade. 7

SUMMARY This invention provides an electrolytic process for strippingaluminide coatings from basis metals and alloys of Period IV Group VIIIof the periodic arrangement-of the elements, that is, iron, cobalt, andnickel, frequently referred to as the metals of the iron group. As usedherein, the term iron group includes not only the pure metals but alsoalloys wherein the major matrix, that is, more than 50 percent, iscomposed of one or more of the members of Period IV Group VIII.

The present process employs oxidizing acid solutions and electricalpotentials such that the basis metal is passivated wherever it becomesexposed to the acid solution and thus does not react or go intosolution, while the reaction with the coating continues at portions notyet fully stripped until all the aluminide coating has been removed,including intergranular penetration and diffusion of the aluminum intothe basis metal. Completion of the process is readily visible by changein color of the stripped article.

It is therefore an object of the present invention to provide a processfor stripping aluminide coatings from basis metals and alloys of theiron group.

It is another object to provide a process for stripping aluminidecoatings wherein the basis metal is not deleteriously affected by theprocess.

A further object is to provide an electrolytic process for strippingaluminide coatings in an oxidizing acid electrolyte and at such apotential that the basis metals is passivated and does not react withthe electrolyte.

Other objects and advantages will become apparent on reading thefollowing specification in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a representation of themetallurgical structure of a superalloy of the iron group bearing analuminide coating; 1

FIG. 2 is a representation of the same alloy after removal of thealuminide coating by the process of the invention;

FIG. 3 is a similar representation, on a somewhat larger scale of thegrain structure, of a superalloy of the iron group after having had thecoating removed by the process of the prior art, showing damage to thebasis metal;

FIG. 41 is a cross-section of a turbine blade after removal of thecoating by the process of the invention;

FIG. 5 is'a cross-section similar to FIG. 4, showing damage to thetrailing edge of a blade stripped by the method of the prior art; and

FIG. 6 shows a semischematic arrangement for practicing the process ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Basically, the processcomprises the following steps:

1. Degreasing. The articles to be stripped are immersed in a degreasingfluid of the solvent type to remove oil, grease, waxes, resins, andother surface accumulations which are soluble in the fluid.Trichloroethylene, perchloroethylene, and trichloroethane are examplesof suitable degreasing fluids of the solvent type.

2. Grit Blasting.- The degreased articles are then cleaned of anyremaining surface deposits by blasting with grit, such as aluminumoxide, in grains of appropriate size. Grit of sieve sizes No. 100 toabout No. 200 are suitable, with about No. 120 considered preferable.Blasting may be done at gauge pressures of about 20 to about psi, with40-60 psi preferred.

3. Electrolytic Stripping. The parts are immersed in a solution of anoxidizing acid maintained at room temperature, from about 60 to aboutwith the parts connected as anodes to a source of electrical power at apotential not higher than 2.4 volts. The other pole of the power sourceis connected to lead cathodes immersed in the solution, lead having ahigh resistance to the acids used. The lead cathodes should have anaggregate surface at least equal to that of the parts being processed,and preferably a greater surface. A convenient mode of assuring adequatelead surface is to carry out the process in a lead-lined tank. Thedistance separating the anodic parts and the lead cathode may varyconsiderably, with the reaction taking more time as the distance isincreased. Where the articles treated are of highly irregular shape suchas turbine blades, and hence cannot present surfaces at equal distancesfrom the cathode, a general separation of about 4-6 inches between theparts and the cathode is suitable for reasonably even stripping of theparts.

Suitable oxidizing acids are sulfuric acid and phosphoric acid, at aconcentration from about 40 percent to about 60 precent, preferablyabout 50 percent. Agitation of the solution is provided by anyconvenient means such as mechanical stirring, oscillation of the meansby which the parts are suspended, or by a pump which circulates thesolution.

Stripping is continued until all the aluminum is removed, which isreadily discernible by the color of the parts. The aluminum or aluminidephase has a gray color which is markedly distinct from the tan orbrownish color of basis metals of the iron group when they becomeexposed.

The time required to completely strip the parts is subject toconsiderable variation, depending on such factors as temperature,voltage, the concentration of the acid solution, the thickness of thealuminide coating, the kind of basis metal, and the distance betweenelectrodes. At a temperature of about 75F with fresh solution, analuminide coating about 0.005 inch thick, and a distance of 4-6 inchesbetween electrodes, the stripping time at a potential of approximately 2volts is from about 5 hours to about 7% hours for cobalt basis metals.Nickel basis metals and iron basis metals under the same conditionsstrip in about 2 to 2% hours. With thinner aluminide coatings the timesare correspondingly decreased. However, there is no adverse effect froman overrun of time, owing to the passivation of the exposed basismetals, as will be explained below. Portions of the parts which may beclosest to the lead cathode will be stripped before the remainder, butthe reaction will proceed with only the remaining coating withoutaffecting the exposed basis metal.

4. Rinse. When the aluminide coating has been completely stripped, theparts are removed from the stripping solution and rinsed in cold runningwater for a minimum of one minute, or in several changesof cold water.

5. Scrubbing. Some light smut or sludge may remain on the parts afterrinsing. In this case they are scrubbed with a suitable abrasive powder,such as pumice, and a stiff brush. If parts are being processed on acontinuous basis with a large throughput, they may be run through anautomatic scrubbing machine with rotating brushes appropriatelyoriented, or tumbled in an abrasive slurry, or handled by oth ecknownproduction means for scrubbing.

6. Final Rinse and Dry. The parts are again rinsed in cold running wateror several changes of water, and may then be allowed to air dry.However, for faster processing it is preferable to follow the cold waterrinse with a hot water rinse to heat the parts, and then blow them drywith air, either individually or by running them through a blowerchamber. The parts are then ready for inspection, and those which havesuffered no deterioration in service may be realuminided and returned toservice.

In FIG. 1 there is shown the metallurgical structure of an articleformed of a superalloy of the iron group containing chromium as one ofthe alloying elements, in a cross-section at the surface and transversethereto. The basis metal 11, a nickel alloy, is shown with its grainstructure and having a two-phase coating. The phase 12 is in thediffusion zone immediately adjacent the basis metal, and comprisesprincipally nickel aluminide and some carbides of the basis metal. Asub-phase of elongated fingers l3 projecting from the basis metal intothe aluminide is formed of chromium-rich material. The formation ofthese chromium-rich fingers is due to the diffusion of aluminum into thebasis metal, which lowers the solubility of chromium in the nickel ofthe alloy, causing the precipitation of chromium out of solution in thisregion. The surface phase 14 comprises practically pure aluminum.

FIG. 2 shows a similar cross-section of the same article after strippingthe coating according to the process of the invention. The aluminumphase 14 and the aluminide phase 12 have been removed, and thechromium-rich fingers 13 have crumbled and disappeared, leaving thebasis metal 11 exposed, without attack by the stripping solution.

FIG. 3 shows a similar cross-section of a basis metal 1 la of the irongroup after stripping an aluminide coating by a process of the priorart. The surface 16 has been attacked by the stripping solution, leavinga pebbled appearance when viwed in natural size. There has also beenintergranular attack, leaving fissures 17 at the surface which are areasof weakness and offer starting points for eventual cracking of the part.

FIG. 4 is an enlarged cross-section of the airfoil portion of a turbineengine blade 18 fabricated of a superalloy of the iron group, afterstripping an aluminide coating by the present process. The trailing edge19 of the airfoil tapers to a very thin section, varying somewhataccording to the size and design of the engine, but frequently having aterminal radius of no more than- 0.005 inch. Such a trailing edge verynearly amounts ot a knife edge, but nevertheless is not damaged by thestripping process of the invention. It is important that this dimensionof the blade should not be altered, since the throat dimension betweenadjacent blades in an engine is defined by the distance between thetrailing edge of one blade and the convex surface of the blade adjacent.The allowable tolerance on the throat dimension in many engines is onlya few thousandths of an inch.

FIG. 5 shows, in exaggerated form, the diminution of the blade span of ablade 18a stripped by a prior art process, in which the portion 19a ofthe trailing edge shown in dotted line has been dissolved in thestripping solution. Such loss of material has a serious effect on theefficiency of the engine when the blade is reinstalled, because of thechange in dimension and form of the interblade passages and theconsequent undesirable changes in gas flow.

The type of damage shown in FIG. 5 arises from the fact that in usingthe reducing acid stripping solution of the prior art the basis metalwill be attacked wherever exposed, even though stripping may not yet becomplete at other portions of the surface of the object; and this attackwill be accentuated if the exposed portion happens to be a relativelysharp edge, owing to the high concentration of the electrical field atedges and corners, especially with the higher voltages of the prior art.

Since turbine blades have a very complex geometry and it is impossibleto position such a blade in the solution in such a manner that all partsof its surface will be at equal distances from the cathode, it isinevitable that the closest portions will be stripped first of theircoating. The basis metal at such a first-stripped portion, if a reducingacid is employed, will begin to react and will be dissolving while theremainder of the article is being stripped. If the closest portionhappens to be a thin section such as a trailing edge, part of it mayentirely disappear before the remainder of the blade is stripped, whenusing the reducing acid and high voltage of prior art processes.

With the oxidizing acids of the present process and a potential nothigher than 2.4 volts, exposed basis metals and alloys of the iron groupbecome passivated and do not react with the electrolyte. Chromium alsoparticipates in this characteristic, so that although it isjone of theprincipal alloying elements of superalloys of the iron group itspresence does not diminish the suitability of such superalloys fortreatment by this stripping process.

Anodic polarization of the metals and. alloys ofthe iron group and ofchromium results in the passivation of these metals. Passivity isattributed to the formation of metal-oxygen complexes at the anodesurface which make the signal potential of the anode more positive andhinder its solution. Such metal-oxygen complexes may occur either by theformation of a very thin film.

(micron thicknesses) of the metal oxide on the surface of the metal, orby the formation of a solid solution of the oxide in the. anode metal atthe surface.

Such passivity of the anode metal forming the object to be stripped canonly be maintained at low voltage, 2.4 volts having been found to be thehighest potential at which the process can be practiced. At higherpotentials the driving force ofthe voltage will dislodge the oxide filmor the solid solution of oxides, and the basis metal will be propelledtoward the cathode. However, the only lower limit is volts. Thedisadvantage of operating at a lower potential than necessary is thatthe time of stripping is increased. A convenient practical potential forpracticing the process on the metals and alloys of the iron group,whether containing chromium or not, is 2 volts.

' It should be pointed out that the process is not characterized interms of current density, the operative conditions being the potentialdifference between the anode and cathode, in a solution of an oxidizingacid. Hence, precise spacing between the anode and cathode is notcritical. Although a spacing of 4-6 inches has been given as aconvenient distance when stripping turbine blades, it was chosen topromote relatively even stripping of the complex surfaces of the blades.The distance could be considerably increased with the only effect beingincrease in the time required. Further, if the articles to be strippedhave flat surfaces which can be opposed to flat cathodes, or if anodeand cathode have some other configuration allowing parallelism of theirrespective opposed surfaces, the separation between the electrodes canbe greatly diminished and the processing time shortened.

FIG. 6 shows an embodiment of equipment suitable for stripping articlesaccording to the process of the invention. A tank 21 of any suitablematerial is provided with a lead lining 22. The tank bears appropriatesupporting means 23 for positioning an anode rod 24-or other anode meansabove the tank. Althoughthe supporting means 23 is here shown asbrackets attached to the tank shell, the anode rod may equally well besupported and positioned by any convenient means, not necessarilyattached to the tank. The power supply for the process may be batteriesputting out an appropriate voltage, or may be the available line currentwith a variable voltage re ctifier 26 interposed. The positive output isconnected to the anode means 24 and the negative terminal to the leadlining 22 of the tank. The tank is filled with a solution of anoxidizing acid 27, for which an agitating means 28 is provided, whichmay be a circulatory pump as shown or other appropriate means. i

A plurality of articles to be stripped of aluminide coating, in thiscase turbine blades 18, are suspended immersed in the electrolyticsolution of oxidizing acid, from metal brackets 29 which are hooked onthe anode rod 24. It is' to be understood that the suspension means mayhave any form adapted to the articles to be processed, as long as it isa conductor which makes the anode connection. Following are examples ofthe practice of the process on aluminidedturbine blades.

EXAMPLE 1 The articles to be stripped were turbine stator blades havinga basis metal formed of a superalloy of the iron group known as lnconel713C, a trademark of the International Nickel Company, having thefollowing nominal composition:

Carbon .12%

Chromiun 13.00

Aluminum 6.00

Molybdenum 4.50

Columbium plus Tantalum 2.00

Titanium .60

Manganese 0.50

Silicon 0.50 max.

Cobalt 1.00 max.

Iron 2.50 max.

Nickel Balance The blades had an aluminide coating of approximately0.005 inch in thickness. They were degreased in trichloroethylene forapproximately 5 minutes, with agitation. The blades were then blastedwith No. grit at a gauge pressure between 40 and 60 psi until theyappeared visually clean of any surface deposits.

The blades were then suspended in a lead-lined tank containing asolution of about 50 percent by volume of sulfuric acid at roomtemperature, with the blades connected as anodes and the lead liningconnected as cathode to a power supply maintained at 2 volts. Relativeagitation between the blades and the solution was provided. Within onehour the gray color of aluminum began to disappear from portions of theblades and the brownish color of the basis metal (chiefly nickel) beganto appear. After two hours the blades showed the brownish color of thebasis metal throughout, but the stripping procedure was continued foranother half hour as a precautionary measure.

The parts were then removed from the electrolyte and rinsed in coldrunning water for a minimum of one minute to remove the acid. They werethen scrubbed with pumice and a stiff brush until the metallic color ofthe predominantly nickel basis metal appeared. They were then furtherrinsed in cold running water to remove any traces of sludge, dipped inhot water, and blown dry with air. Subsequent metallurgical examinationshowed that all traces of the aluminide coating had been removed.

EXAMPLE H The parts to be stripped were sections cut from airfoilportions of turbine blades formed of lnconel 713C having the samecomposition as that discussed in Example I. The surface of the airfoilshad an aluminide coating of about 0.003 inches in thickness; however,where the blades were, cut in two the basis metal was exposed from thebeginning.

The parts were degreased and grit-blasted as in Example l. Thereupontheywere suspended in the tank containing a solution of about 50 percentby volume of phosphoric acid, with the parts connected as anodes at apotential of 2 volts. The exposed basis metal of the cut portions tookon the typical brownish hue within a few minutes. Within less than anhour the gray color of aluminum had disappeared from considerableportions of the airfoil surface, being replaced by the brownish hue ofthe basis metal. After an hour and a half the removal of the aluminidecoating appeared visually complete, but the parts were left in thesolution for another half hour.

The parts were then rinsed and cleaned as previously described.Metallurgical examination showed no attack of the basis metal, even ofthe portions which had been exposed before processing, and the aluminidecoating had been entirely removed.

EXAMPLE Ill The articles to be stripped were turbine blades having abasis metal formed of a supe r alloy of the iron group known as HS 31,or Stellite 31, a trademark of the Haynes Stellite Company, having thefollowing nominal composition:

Carbon .50

Chromium 25.00

Nickel 10.00

Manganese .60

Silicon .60

Iron 1.00

Tungsten 8.00

Cobalt Balance The parts had an aluminide coating about 0.005 inches inthickness. They were degreased and gritblasted as in Example I, thensuspended in a lead-lined tank containing a solution of about 50 percentby volume of phosphoric acid with the parts connected to the powersupply as anodes and the lead as cathode, with the potential beingmaintained at 2 volts, and agitation of the solution provided. Thebrownish color of the basis metal, chiefly cobalt in this embodiment,began to appear after a few hours, and on some articles the gray colorof aluminum could no longer be discerned after five hours. However, somearticles still bore traces of aluminum which did not entirely disappearuntil after seven hours of processing. The parts were then left in thesolution under voltage for a further half hour as a precautionarymeasure. They were then removed from the electrolyte, rinsed, scrubbed,again rinsed, and dried as in previous examples. Metallurgicalexamination revealed complete removal of the aluminide coating withoutany attack on the basis metal.

EXAMPLE lV Sections were cut from the airfoil portion of turbine bladesof HS 31, having an aluminide coating of about 0.003 inch, and processedas in Example [I], except for using sulfuric acid as the electrolyte.After five hours of treatment the coating appeared to have been entirelyremoved. Processing was continued for a further half hour, and the partsthen cleaned and dried. Examination showed that all the coating had beenremoved, and that the basis metal had not been attacked at any portion.

What is claimed is:

l. A process for stripping the coating from an article formed of a basismetal of the iron group and having an aluminide coating thereon,comprising cleaning the surface of the coating, submerging the articlein an oxidizing acid-having a lead cathode therein, connecting thearticle as anode to the positive pole of a source of electric power andthe cathode to the negative pole thereof, and applying a potentialdifference less than 2.4 volts between the anode and the cathode untilthe aluminide coating is removed from the article.

2. A process as recited in claim 1, wherein the oxidizing acid is at aconcentration from about 40 percent to about 60 percent.

3. A process as recited in claim 2, wherein the oxidizing acid isselected from the group consisting of sulfuric acid and phosphoric acid.

4. A process as recited in claim 3, wherein the acid solution ismaintained at a temperature from about 60 to about 5. A process asrecited in claim 4, wherein the basis metal of the article ispredominantly nickel.

6. A process as recited in claim 5, wherein the basis metal is .Inconel713C.

7. A process as recited in claim 4, wherein the basis metal ispredominantly cobalt.

8. A process as recited in claim 7, wherein the basis metal is Stellite3|.

9. A process as recited in claim 1, wherein exposed basis metal becomespassivated and is not attacked by the oxidizing acid.

Patent No. 3 ;779 ,879 Dated December 18, 1973 Inventods) Bruce E. ScottIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

IN THE SPECIFICATION:

Column 3, line 18, the word "precent should read --percent Column 4,line 34, the word "viwed" should read --viewed- Column 5, line 32, theword "signal" should read -sing1e--.

Signed and sealed this 7th day of May 197A.

(SEAL) Attest:

EDWARD l-LFLETCHERJR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents FORM P0405) USCOMM-DC 60376-P69 Ui5. GOVERNMENT PRINTING OFFICEI," O-JaQ-SSI

2. A process as recited in claim 1, wherein the oxidizing acid is at aconcentration from about 40 percent to about 60 percent.
 3. A process asrecited in claim 2, wherein the oxidizing acid is selected from thegroup consisting of sulfuric acid and phosphoric acid.
 4. A process asrecited in claim 3, wherein the acid solution is maintained at atemperature from about 60* to about 90* .
 5. A process as recited inclaim 4, wherein the basis metal of the article is predominantly nickel.6. A process as recited in claim 5, wherein the basis metal is Inconel713C.
 7. A process as recited in claim 4, wherein the basis metal ispredominantly cobalt.
 8. A process as recited in claim 7, wherein thebasis metal is Stellite
 31. 9. A process as recited in claim 1, whereinexposed basis metal becomes passivated and is not attacked by theoxidizing acid.